In the electrophotographic process, an image of light and shadow areas is optically projected onto a charged photoconductor. The charge on the photoconductor dissipates in regions which are illuminated, while the charge is retained in those regions of the photoconductor which are not illuminated. The development step consists of the electrostatic attraction of charged toner particles to the electrostatic latent image on the photoconductor surface.
Several techniques have been developed for the development of the electrostatic latent image. The most common methods use relatively large carrier particles (about 50-1000 microns) to bring the smaller toner particles (about 1-50 microns) near the photoconductor surface. The carrier and toner particles are carefully chosen so that the toner particles will triboelectrically adhere to the surfaces of the carrier particles. The sign and magnitude of the triboelectric couple between the carrier and toner particles must be carefully chosen so that the toner particles will be electrostatically transferred only to the charged areas of the photoconductor. If the toner is transferred to the discharged areas of the photoconductor, objectionally high areas of background will result. If the surface of the carrier and toner are not each homogeneous, a range of triboelectric couples will be produced. Such a range may result in toner being deposited in inappropriate areas and reduced image quality.
Cascade development, as disclosed in U.S. Pat. No. 2,618,552, utilizes a two component carrier/toner system. The two component developer is poured or cascaded over the charged surface of the photoconductor.
U.S. Pat. No. 2,874,063 describes a method for developing an electrostatic image utilizing a magnetic brush. Ferromagnetic carrier particles tend to align themselves into long fibers or bristles under the influence of a magnetic field. Toner which triboelectrically adheres to the carrier can be transferred to the charged photoconductor by passing the magnetic brush over the surface of the photoconductor. The toner transfers to the photoconductor while the carrier remains attached to the brush.
Both the cascade and magnetic brush development methods subject the developer mix to violent agitation, rubbing, and abrasion. Under such conditions the toner particles gradually fragment and form physically adherent films on the carrier surface. As these films cover increasingly larger areas of the carrier surface, more toner-toner contact takes place with a consistent decrease in carrier-toner contact. This has the effect of changing the carefully engineered triboelectric couple and the developer eventually fails.
If the carrier is coated with an organic resin, the resin can crack and peel off. The carrier then presents at least two surfaces to the toner: carrier and pure resin. The toner can then be tribocharged by either of the two exposed carrier surfaces. Failure of the developer mix occurs because the triboelectric charge generated from the resin coating-toner contacts is different than the triboelectric charge developed from the uncoated carrier-toner contacts. The resulting range of triboelectric charge on the toner particles may result in toner being deposited on the photoconductor in inappropriate areas, thereby reducing print quality. The coated carriers may also fail due to buildup of toner films on the surface.
In both cascade and magnetic brush development, the developer is in intimate contact with the photoconductor surface. The developer is abrasive under these conditions and eventually wears out the photoconductor, which then must be replaced.
Developer mixes, particularly those with insulative carriers, often give poor fill-in of solid areas. Development electrodes are often required to overcome this problem. It is sometimes possible for a conductive carrier to act as its own development electrode or to improve the performance of an existing development electrode.
In both cascade and magnetic brush development, the developer is in motion. It is also advantageous to continually mix the developer with replenishment toner, thereby replacing toner removed from the system by the act of development. Both of these procedures are facilitated if the developer has good flow characteristics.
Coating carriers to give improved developers is known in the prior art. Uncoated carrier particles will be called carrier cores in this description. There are several advantages to coating carrier cores. Coated carriers often present a more uniform surface than uncoated cores. Coatings make possible the adjustment of the triboelectric couple with a given developer to a desired value. Coatings also can extend the lifetime of a developer mix. Some examples of coatings consisting solely or in part of organic fluoropolymers are given in U.S. Pat. Nos. 4,147,834; 3,947,271; 3,922,382; 3,918,968; 3,873,356; 3,873,355 and 3,778,262. These patents are primarily concerned with techniques for achieving negatively charged toner from fluoropolymer coated carriers and in extending the lifetime of the developer mix.
Additives are sometimes added to developer mixes instead of actually coating the carrier in an attempt to achieve similar benefits. U.S. Pat. No. 4,248,950 discloses that MoS.sub.2, WS.sub.2, TiS.sub.2, PbO, and graphite are useful additives for prolonging the lifetime of the developer mix.
Carbon fluoride (fluorinated carbon) was reported as an additive to a fluoropolymer carrier coating agent in Japanese Patent Application No.: 1975-64, 590 entitled "Carrier Coating Composition for Electrostatic Photography", published Dec. 3, 1976. According to this disclosure, the carbon fluoride is added to a fluoropolymer carrier coating agent to improve its wear resistance. A non-fluorine containing resin may be used as a binder. When used as an "additive" in this manner, according to this disclosure, the amount of carbon fluoride employed does not exceed the weight % of the fluoropolymer coating agent employed. In other words the effective coating agent is the fluoropolymer; the carbon fluoride is merely an additive.
Graphite fluoride (fluorinated carbon) was also reported as an additive to both toner and the bulk developer mix in U.S. Pat. No. 4,141,849. The main purpose of the fluorinated carbon addition according to this patent is to provide stable charge control to the developer, enhance fluidity of developers and prevent toner from fusing to a photosensitive member. Japan Kokai Tokkyo Koho 78,147,542 describes a similar use of fluorinated carbon in conjunction with fibrous PTFE to afford toners with improved frictional and mechanical properties.
The object of the present invention is to provide a novel electrophotographic carrier which offers some practical advantages over prior art carriers and the use of such carriers in electrophotographic processes.
Other objects and advantages will be apparent from the following description.